1. Field of the Invention
The present invention relates generally to a vibrating rate gyro that can be used to stabilize or guide vehicles, for example, or for automobile navigation.
The invention relates more particularly to a monolithic vibrating rate gyro structure comprising a fixed part, a resonator and a mechanical system linking the resonator to the fixed part and preventing leaks of vibratory mechanical energy from the resonator to the fixed part.
2. Description of the Prior Art
The resonator is intended to operate in two different vibration modes, the vibration directions of the two modes being mutually perpendicular. Accordingly, when the rate gyro is rotating at an angular speed {right arrow over (xcexa9)} about an axis perpendicular to these two vibration directions, this rotation {right arrow over (xcexa9)} being referred to a so-called xe2x80x9cGalileanxe2x80x9d inertial frame of reference, coupling occurs between the two vibration modes of the resonator. The coupling is due to Coriolis accelerations {right arrow over (xcex3)}c that apply at all material points of the resonator and whose analytical expression can be written in the form of the following vector product:
{right arrow over (xcex3)}c=2{right arrow over (xcexa9)}xcex9{right arrow over (v)},
{right arrow over (v)} being the speed of the material
point concerned, expressed in the frame of reference tied to the resonator.
There are several ways to exploit this coupling to measure the angular speed {right arrow over (xcexa9)}.
For example, one of the two vibration modes of the resonator can be maintained in vibration. The rotation at speed {right arrow over (xcexa9)} then causes vibration in the other mode, with an amplitude proportional to xcexa9. The value of xcexa9 can be determined from the measured amplitude, generally converted to the form of an electrical signal.
A second way to exploit the coupling between the two vibration modes of the resonator requires the resonant frequencies of the two modes to be identical, and consists of maintaining them in vibration simultaneously so that one of the modes is in phase quadrature relative to the other mode. Accordingly, if the vibration amplitudes are equal for the two modes, each material point of the resonator traces out a circle in each period of the vibration. The angular frequency xcfx890 of that vibration, observed in an inertial frame of reference, is not affected by the rotation at angular speed {right arrow over (xcexa9)}. On the other hand, the angular frequency xcfx89 of the same vibration, as observed by measuring means tied to the resonator, depends on xcexa9, as expressed by the equation xcfx89=xcfx890xc2x1xcexa9. Thus an oscillator whose frequency variations are representative of rotation speed variations can be made.
A third way to exploit the coupling between the two vibration modes of the resonator requires the resonant frequencies of the two modes to be identical, as before, and consists of maintaining them in vibration simultaneously so that the two modes are in phase relative to each other. Accordingly, the vibrations of each material point of the resonator are contained in a plane. If the system for maintaining the vibrations is designed not to give preference to any particular orientation in that plane relative to the resonator, then the initial orientation, as observed in an inertial frame of reference, is not affected by the rotation at the angular speed {right arrow over (xcexa9)}. On the contrary, the same vibration plane, as observed using means linked to the resonator, undergoes the opposite rotation to that imposed on the resonator. Unlike the first two ways of exploiting the coupling, this third way does not enable the rotation speed xcexa9 to be measured directly because the sensor provides the orientation xcfx86 of the vibration plane. This operation in gyroscope mode is beneficial in some applications, and also enables the speed to be determined by means of a differentiator circuit (xcexa9=dxcfx86/dt).
For each of the above three ways of exploiting the coupling between the two vibration modes of the resonator, the accuracy of the measurement supplied by the rate gyro depends on the quality of the vibrations of the two modes. To be more precise, for each of the modes, it is preferable for the quality factor Q to be high, enabling mechanical phenomena resulting from Coriolis accelerations to be amplified. For one resonator the Q-factor is defined as the quotient of the energy stored in the resonator divided by the energy loss during one period of the vibration. The Q-factor depends on a plurality of parameters. Firstly, the nature of the material used to fabricate the resonator conditions the mechanical energy losses by internal friction. For this reason the material is advantageously quartz or silicon, for which these losses are small, and which further have the benefit of a low acquisition cost. A second parameter influencing the Q-factor is the nature of the vibration of the resonator. For example, if the resonator is a beam vibrating flexionally, the Q-factor depends on exchanges of heat between the fibers that are alternately stretched and compressed, and this value is therefore proportional to the square of the thickness of the beam. These first two parameters determine what might be called the xe2x80x9cintrinsicxe2x80x9d quality factor Q of the resonator. In practice, the real Q-factor of the resonator is at most equal to this intrinsic factor, because it is generally affected by other influencing parameters, in particular the means of fixing the resonator to a support. It is desirable for the fixing means to avoid mechanical energy leakages from the resonator to the support. To be more precise, it is desirable to limit these energy leakages to a level related to the intrinsic quality factor of the resonator. For example, to obtain the greatest benefit from a resonator whose intrinsic quality factor is 3xc3x97105 , it is necessary for the fixing means to limit energy leakages in each period to a value significantly lower than (3xc3x97xc3x97105 )xe2x88x921 times the energy stored in the resonator. To this end, the fixing means can consist of a fixed part and a mechanical system connecting the resonator to the fixed part and reducing sufficiently leakages of mechanical energy from the resonator to the fixed part. The device therefore performs a function of mechanically filtering vibrations of the resonator. The fixed part is therefore little affected by the vibrations of the resonator. Accordingly, when the fixed part is fixed to a support, for example glued to the support, the mechanical energy leakages to the support can be sufficiently small. The resonator, the fixed part and the mechanical filtering system constitute a rate gyro structure.
The rate gyro structure is preferably monolithic, which avoids the drawbacks inherent to assemblies of several components, such as behavior that is unstable in time or in the event of temperature variations.
The monolithic structure of the rate gyro is preferably machined from a plate of material of uniform thickness, which enables it to be fabricated at low cost by chemical machining.
The resonator is preferably a tuning fork formed of two identical and parallel branches facing each other and each fixed at one end to a common part. One of the vibration modes is a flexional mode, in which the two branches vibrate flexionally in phase opposition, the vibratory displacements of the two branches being parallel to the plane of the plate of material. This vibration mode is therefore similar to that of a musical tuning fork. The other vibration mode is a torsional mode, in which the two branches vibrate flexionally in phase opposition, the vibratory displacements of the two branches occurring perpendicularly to the plane of the plate. The vibration directions of the two modes are therefore mutually perpendicular, and coupling occurs between the two modes if the tuning fork is subjected to rotation about an axis parallel to the two branches. That axis is called the sensitivity axis of the rate gyro. The benefit of choosing a tuning fork as the resonator is that the mechanical energy remains spontaneously localized in the resonator for the bending mode, the vibratory mechanical forces exerted by the two branches where they are xe2x80x9cbuilt intoxe2x80x9d the common part balancing out in the area of that common part near these build-ins. For this flexional mode, the mechanical energy leakages can therefore be reduced sufficiently by fixing the tuning fork to a support in an area of the common part sufficiently far away from the build-ins of the branches. On the other hand, this type of fixing would not be suitable for the torsional mode, because the support would be loaded by a torque alternating at the frequency of the torsional vibration and this would lead to high leakages of mechanical energy to the support.
To solve this problem of mechanical energy leaks in the torsional mode of the tuning fork, U.S. Pat. No. 5,166,571 proposes an H-shaped rate gyro structure which can be considered as consisting of two tuning forks sharing the same common part. FIGS. 1A and 1B show the greatly enlarged vibrational deformations of this structure for the flexional and torsional modes, respectively. From the theoretical point of view, the concept is beneficial because it seeks to have the vibrations of the four branches balance out for each of the two modes. In practice this solution has a drawback relating to the fixing of the structure, because the common portion 30a is subjected to flexional and torsional vibratory deformations which prevent it from being fixed directly to a support without causing high leakages of mechanical energy. Using the H-shaped structure therefore calls for additional mechanical retaining means, such as the fixing projecting part 78 shown in FIG. 1C, which has the drawback that it no longer benefits from the quality and low cost of manufacture of a plane monolithic device.
Still with the aim of solving the problem of mechanical energy leakages in the torsion mode of the resonator, it might seem beneficial to take inspiration from the monolithic quartz acceleration transducer structure proposed in the assignee""s French patent No. 2,739,190 and shown in FIG. 2A. Unlike the known rate gyro previously described, this transducer is designed to measure acceleration and must therefore be relatively insensitive to rotation. From the structural point of view, the transducer shown in FIG. 2A has a monolithic structure comprising a fixed part 1, a resonator 3 fastened at both ends to a mobile mass part 2 serving as a test mass and a second mass part 4, respectively. First connecting means each connect the first mass part to the mobile second mass part and consist of two short blades 81 and 82 (H8 less than  less than H3) with a high flexural stiffness. The blades are vertically aligned with the middle of the length of the resonator 3 and the mobile parts 2 and 4 are consequently U-shaped. Also, the blades 81 and 82 have a thickness E8 significantly less than the thickness E of the plate. The blades 81 and 82 are equivalent to relatively rigid hinges between the mobile parts in the plane of symmetry PS containing the sensitive direction perpendicular to the plate and rigid connections in the plane PM of the plate. Second connecting means each connect the second mobile mass part to the fixed part and consist of a flexible frame 5 around the mass parts 2 and 4, a first connecting member 6 connecting the frame 5 to the second mass part 4, and a second connecting member 7 connecting the frame 5 to the fixed part 1. The fixed part is fixed to a housing base BA. The resonator 3 is a parallelepiped-shaped blade preferably vibrating flexionally or torsionally, because the frequencies of such vibrations are highly sensitive to the tensile or compression force exerted longitudinally on the resonator when the test mass 2 is subjected to an acceleration. Measuring the frequency of the resonator 3 by means of an appropriate device (not shown) therefore determines the acceleration applied to the transducer. This sensitivity of the frequency of the resonator to longitudinal forces is enhanced if the thickness of the parallelepiped-shaped blade constituting the resonator is reduced. In contrast, this reduction in the thickness of the blade reduces its intrinsic quality factor. Seeking a satisfactory compromise leads to an intrinsic quality factor of the order of 104, a relatively modest value for a quartz resonator. FIG. 2B shows the greatly enlarged vibratory deformation of the structure of this transducer when the resonator is vibrating torsionally about its longitudinal axis. As explained in the French patent No. 2,739,190 already cited (page 22, line 6-page 23, line 13), the rotational inertia of the mass parts 2 and 4 and the torsional flexibility of the flexible frame 5 characterize a mechanical filter between the resonator 3 and the fixed part 1 of the transducer; said fixed part is only very slightly loaded by the vibrations of the resonator. The alternating forces acting on the fixed part consist primarily of a torque t whose intensity is very much less than that of the torque T applied by the resonator to each of the mass parts 2 and 4. This limits energy leakages in each period of the vibration to a value significantly less than 10xe2x88x924 times the energy stored in the resonator. Accordingly, as previously explained, the efficacy of the mechanical filtering performed by this device is well suited to the relatively modest intrinsic quality coefficient of the resonator, as a result of which the acceleration transducer works well. However, this mechanical filtering device would not be satisfactory for a rate gyro structure in which, as previously explained, it is desirable for the intrinsic quality coefficient of the resonator to be high, for example equal to 3xc3x97105 . In this case, it is therefore desirable for the mechanical filtering device to limit energy leakages in each period of the vibration to a value significantly lower than (3xc3x97105)xe2x88x921 times the energy stored in the resonator. If the masses and the stiffnesses of the device shown in FIGS. 2A and 2B were adapted to obtain this significantly increased efficacy, this would lead to disadvantages concerning the difficulty of defining and producing the rate gyro structure, as explained below. In the French patent No. 2,739,190 already cited, it is indicated (page 23, lines 29 to 36) that the mechanical filtering device is effective at the frequency of the resonator (a few tens of kHz) and that the mechanical strength of the transducer is not degraded in the working pass-band (from D.C. up to a few hundred Hz). This indicates that the frame 5 shown in FIGS. 2A and 2B is sufficiently flexible at a few tens of kHz and sufficiently rigid at a few hundreds of Hz, and also that the mechanical filtering device can be considered in a simplified way as a filtering suspension whose resonant frequency Fs is a few kHz, around ten times less than the frequency F of the resonator. The suspension frequency Fs is the lowest resonant frequency of the transducer structure. Because of its relative complexity, the structure has a number of resonant frequencies between Fs and F, and the density of the spectrum of these resonant frequencies increases with the frequency values. Correct operation of the resonator requires avoiding a close spacing between its resonant frequency F and a structure resonant frequency. The lower the frequency F, the easier it is to comply with this constraint. On the other hand, the efficacy of the filtering suspension increases as the frequency F increases, as indicated by the following approximate equation relating the total alternating torque 2T applied by the resonator to the mobile assembly consisting of the mass parts 2 and 4, and the alternating torque t received by the fixed part 1:
t/2T≈Fs2/F2
In practice, it is desirable for the frequency F of the resonator not to exceed about ten times the lowest frequency Fs of the structure, because the geometry of that structure can then be defined with manufacturing tolerances that are sufficiently large to be compatible with a low fabrication cost. This condition is satisfied for the acceleration transducer shown in FIGS. 2A and 2B, for which the torque t received by the fixed part is therefore around 100 times smaller than the total torque 2T applied by the resonator at its xe2x80x9cbuild-insxe2x80x9d. Regarding the use of this type of filtering suspension in a rate gyro structure, however, it will have been understood that the need for significantly increased efficiency would lead to the resonator being operated at a frequency F significantly greater than ten times the lowest frequency Fs of the structure; the consequences of this would include problems with designing and fabricating the rate gyro structure and therefore an increase in its fabrication cost.
The present invention proposes a geometrical form for a monolithic rate gyro structure which, whilst sufficiently limiting vibratory mechanical energy leaks from the resonator to the fixed part, is better suited to the industrial requirements for high-performance low-cost rate gyros.
According to the invention, this monolithic rate gyro structure, comprising a fixed part, a resonator, a first mobile mass part fastened to one end of the resonator, a second mobile mass part, first connecting means connecting the first mobile mass part to the second mobile mass part, and second connecting means connecting the second mobile mass part to the fixed part, is characterized in that the second mobile mass part is not fastened to the resonator and is situated in the vicinity of the other end of the resonator, the first connecting means comprise first two flexible arms which extend along the whole of the resonator on respective opposite sides thereof, and the second connecting means comprise second two flexible arms which extend along the first flexible arms and connect the second mobile mass part directly to the fixed part.
This disposition of masses and stiffnesses between the resonator and the fixed part achieves sufficiently effective mechanical filtering whilst causing the resonator to operate at a frequency F that does not exceed about ten times the lowest frequency Fs of the structure. This facilitates designing and fabricating the structure and reduces its fabrication cost.
According to a preferred embodiment, the fixed part consists of two parts situated along edges of the second two flexible arms which are opposite edges of the second flexible arms facing the first two flexible arms.
Preferably, the resonator is a tuning fork formed of two identical and parallel branches facing each other and each fastened at one end to a common part, the first mobile mass part being fastened to the resonator in the vicinity of the area of said common part at the greatest distance from said branches.
The rate gyro structure has an axis of symmetry to obtain the maximum efficiency of mechanical filtering.
Preferably, the lowest frequency of vibration of resonance of the gyro structure about said axis of symmetry is substantially equal to the lowest frequency of vibration of resonance of said structure, and the frequency of the resonator is equal to approximately ten times said lowest frequency of vibration of resonance of said structure.